Field of Invention
This invention pertains generally to inline sensors and measurements and, more particularly, to an air cooled ultraviolet (UV) light source for use in such applications.
Related Art
Inline UV sensors are widely used in industrial and pharmaceutical applications. Heretofore, low and medium pressure gas discharge lamps have been used as the light source in making such measurements. In pharmaceutical applications, the light source most commonly used is a low pressure mercury (Hg) lamp which emits discrete lines of light at a plurality of specific wavelengths in the UV and visible spectrum. To isolate specific emission lines or wavelengths of interest, bandpass interference filter must be used. Such filters greatly reduce the available optical signal, particularly in the UV spectrum where the transmission of the filters rarely exceeds 20 percent.
The low pressure Hg lamp assemblies commonly used for inline sensor applications typically require approximately 4-5 watts of power, and the overall efficiency is low and distributed across the many discrete spectral lines emitted by the lamp. These lamps exhibit optical noise and tend to drift in output, dissipate heat due to operation at high envelope temperatures, and require special high voltage power supplies.
To compensate for variations in lamp output and maintain accurate results, measurement signals and reference signals are monitored and compared. To isolate a spectral line of interest, matched filters must be used for both the reference signals and the measurement signals. In dual beam applications, the optical requirements are further complicated since matched filters and detectors are required for each reference and measurement channel, and the channels must be isolated. This can be done, for example, either by the use of a side-by-side filter/detector configuration or by the use of beam splitting, either of which will result in a further reduction of the optical signals.
If the desired wavelength is not one of the available emission lines, the lamp is coated with a phosphor which fluoresces at the desired wavelength when excited by one of the available lines, and efficiency is further reduced.
A typical low pressure Hg lamp assembly of the prior art is illustrated in FIGS. 1-2. This assembly has a cylindrical housing 11 with an electrical connector 12 at one end and a fitting 13 at the other end for connection to a flow cell. A low pressure Hg lamp 14 and a high voltage power supply 16 are enclosed within the housing along with reference detectors and filters 17, 17. As these drawings illustrate, due to the relatively large sizes of the lamp and its power supply, the overall assembly is also relatively large in size and cumbersome to use.
Another problem with low pressure Hg lamps is that when they are used in hazardous environments, special containment enclosures and connectors must be employed to meet safety requirements. These requirements typically include an approved housing that is capable of containing explosions in the event the gas in the lamp should ignite and special cabling which will isolate electrical signals from the hazardous environments. As illustrated in FIG. 3, such a device typically includes an explosion proof housing 19 with an explosion proof end cap 21 and an explosion proof cable gland 22 at one end and an explosion proof window assembly 23 and a flow cell adapter 24 at the other end.